The PalmOS doesn't have as much overhead as the PocketPC operating system. PalmOS only has to be designed to run on ONE processor, the DragonBall. The PocketPC is built to handle three totally different architectures (StrongARM, MIPS, and SH3). This will all change though with Merlin (the next version of the PocketPC operating system). It will only have support for ARM based processors.
There are also other technical matters that lie beneath, but there is no way to really directly compare them as far as speed goes. It's kinda like trying to compare Linux to Windows 2000. They're two completely different operating systems.
That being said, PocketPC's are more than fast enough for what they do and are as fast for the usual PIM duties as Palm devices. The icing on the cake however is that PocketPC's can do SO much more than your typical Palm (video, multimedia, MP3's, 802.11b wireless LAN, etc. etc.). PocketPC's, especially the iPAQ, are making HUGE gains in the corporate market. Even Palm stated just this past week that they have not done enough to make a push into the corporate sector like PocketPC's have.
And considering that a B&W iPAQ 3135 with a 206MHz StrongARM processor can be had for ~$160 now, I can't see the use in getting a B&W PalmOS based device anymore. And with color PocketPC's dropping in price (Casio E125, $399--integrated CF Type-II, 16-bit color, etc.), it's hard to make a case for color Palms as well 😉
